Direct wafer bonding refers to a process where two separate wafer surfaces are brought into contact and are bonded without any intermediate adhesives or external force. The initial bond strength is usually weak, and therefore a subsequent annealing step is generally carried out to strengthen the bond. Wafer bonding is deployed in a wide range of semiconductor process applications including substrate engineering and fabrication of integrated circuits, packaging and encapsulation of micro-electro-mechanical-systems (MEMS) and stacking of many processed layers (3D-integration) of pure microelectronics.
The quality of the wafer bond plays an important role in the overall semiconductor production yield and cost. Therefore, monitoring and characterization of the bond quality during processing and post-processing is desired. Quick and nondestructive bond quality measurement methods are preferred on the wafer level. Examples of nondestructive bond quality measurement methods include acoustical and optical methods. These methods are applied post bonding and measure the quality of the bond in terms of interface defects. A pre-selection of known-good dies is done after singulation, and the bond strength and other parameters like hermeticity are tested in a sample test with destructive methods. In cases where micro-sensors will be used for human safety applications each device must pass a sequence of harsh environmental tests. All these prior art wafer bond testing methods deal with post bonding metrology. However, in many applications it is desirable to monitor the wafer bonding process and bond quality in-situ and in real time. Monitoring of the bonding time in a production type environment is not practiced today due to the lack of technical solutions. Optical methods deploying infra-red (IR) light, as shown in FIG. 1, are typically used for a post bond void inspection. A set up for monitoring of the bonding based on the IR inspection system is incorporated in the ABC200 and the ELAN SOI300 systems manufactured by SUSS MicroTec. These systems however do not incorporate automated bonding and bond time monitoring. Accordingly, there is a need for a system that provides in-situ and in real time wafer bonding time monitoring.